docs: def load_signature(stream, original_data):
      """Load signature from stream, and compute GPG digest for verification."""
      signature, = list(parse_packets((stream)))
      hash_alg = HASH_ALGORITHMS[signature['hash_alg']]
      digest = digest_packets([{'_to_hash': original_data}, signature],
                              hasher=hashlib.new(hash_alg))
      assert signature['hash_prefix'] == digest[:2]
      return signature, digest